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A theoretical model is developed for the forced convection film boilingphenomenon over a heated sphere moving vertically downwards in the water.Unprecedented to the previous analytical studies, this model accounts for thebuoyancy effects while solving the momentum, and energy equations in the vaporphase to obtain the velocity, and the temperature distribution in terms of thevapor boundary layer thickness. To calculate the vapor boundary layer thicknessan energy balance is applied at the vapor-liquid interface. The flow of liquidaround the sphere is considered to be governed by the potential theory, and theenergy equation in liquid is then solved for the known velocity distribution.We find that the vapor boundary layer thickness increases with an increase inthe sphere, and the bulk water temperature, and a decrease in the free streamvelocity. This further results in a decrease in the film boiling heat transfercoefficient. The present study concludes that at low free stream velocities ($<0.5$ m/s) buoyancy becomes significant in delaying the separation, and when thevelocity is further reduced the separation angle approaches $180^{\circ}$.

Buoyancy effects on film boiling heat transfer over a sphere at low velocities